Substantial development work has taken place relating to making electrical contacts to semiconductor devices such as photovoltaic cells by using so-called thin film techniques. Among the more popular thin film techniques are vacuum deposition and sputtering. These methods enable the formation of pure metal, metal mixture and laminated metal contacts to silicon. Pure silver contacts made in this way have yielded excellent electrical properties but questionable adhesion strength. This was improved upon by the interposition of a thin layer of titanium between the silicon and the silver as disclosed in U.S. Pat. No. 3,106,489. Unfortunately, such titanium-silver contacts were found to degrade in a humid environment and this lead to the development of the titanium-palladium-silver contact, details of which are given in U.S. Pat. No. 3,686,036. An improved version of the titanium-palladium-silver contact in which the titanium-palladium layer actually comprises a titanium-titanium/palladium-palladium layer to improve adhesion, and the silver is plated on by electroless deposition, is disclosed in U.S. Pat. No. 4,082,568. The very complexity of the above process suggests that it is impracticable for the needs of solar cells for the 1980's.
Thick film technology, on the other hand, appears to be a more practical alternative to thin film deposition. Thick film compositions containing silver and a dopant material of the n- or p-type have been applied to p- or n-type silicon semiconductor wafers by screen printing techniques. The compositions are then fired. During sintering of the metallization the dopant diffuses into the silicon and serves to enhance the current carrier concentration in the vicinity of the metallization. The addition of dopant is carried out so that the contact resistance of the conductor-forming material is minimized. A contact resistance of less than 10 m.OMEGA.-cm.sup.2 is acceptable. Such thick film technology is described in U.S. Pat. Nos. 4,153,907 and 4,163,678.
In U.S. Pat. No. 4,153,907, the dopant material is phosphorus derived from phosphorus-doped nickel powder for n-type silicon and boron derived from borosilicate glass for p-type silicon. In U.S. Pat. No. 4,163,678, a composition is disclosed comprising silver and silver metaphosphate powders dispersed in an ethyl cellulose-butyl carbitol acetate vehicle solvent system. This composition is applied to n-type silicon. The dopant employed is phosphorus derived from silver metaphosphate.
These two prior art compositions suffer from having a tendency to diffuse too deeply into the n-layer of the solar cell. Diffusion in the proximity of the p-n junction runs the risk of affecting the quality of the solar cell. A practical composition that does not rely on the diffusion of a dopant material component is needed.